docs(theming) update theme.config override example

[the133448]

This matches the format inside node_modules/semantic-ui-less/theme.config.example
fixes #4173

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[codecov-commenter]

Codecov Report

Merging #4174 (e2b70a4) into master (cc7ec31) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4174   +/-   ##
=======================================
  Coverage   99.75%   99.75%           
=======================================
  Files         180      180           
  Lines        3241     3241           
=======================================
  Hits         3233     3233           
  Misses          8        8           

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[the133448]

Thanks @layershifter , your right! Don't know why I missed that.

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉